It is known to produce relatively large electromagnets of superconducting wire for use, for example in magnetic resonance imaging (MRI) systems. Known magnets for MRI systems may be 2 m in diameter, 1.5 m in length and include many tens of kilometers of wire. Commonly, the magnets are composed of several relatively short coils, spaced axially along the axis of a cylindrical magnet, although several other designs are known, and the present exemplary embodiments are not limited to any particular magnet design.
Such superconducting magnets are not normally wound from a single length of superconducting wire. If several separate coils are used, they are usually produced separately and electrically joined together during assembly of the magnet. Even within a single coil, it is often necessary to join several lengths of wire together.
Joints between superconducting wires are difficult to make. Optimally, the joint itself will be superconducting—that is, having a zero resistance when the magnet is in operation. This is often compromised, and “superconducting” joints are often accepted which have a small resistance.
A common known manner of making a superconducting joint is to take the lengths of superconducting wire, and strip any outer cladding, typically copper, from the superconducting filaments from a length at or near their ends.
The superconducting filaments of the two wires may then be twisted together. The resulting twist of filaments is then coiled into a joint cup: a fairly shallow vessel, typically of copper or aluminum.
Alternatively, the filaments may be plaited, rather than twisted, before being coiled into the joint cup.
In other arrangements, the filaments of the wires are simply laid side by side, not necessarily touching one another, and placed within the joint cup.
The superconducting joint is then made as described below.
The present exemplary embodiments accordingly seek improved superconducting joints and methods for cooling superconducting joints to enable the superconducting joints to be sufficiently cooled in magnets which are not cooled by immersion in a liquid cryogen.
In order to manufacture low cryogen inventory superconducting magnets—that is, those which do not rely on cooling by immersion in a bath of cryogen, but are cooled by a reduced volume of cryogen, for example in a thermosiphon or cooling loop—or cooled by solid conduction without the use of cryogens—it is necessary to produce suitably cooled superconducting joints which do not require cooling by immersion in cryogen.
One approach to this problem may be in using flexible thermal conductors such as copper or aluminum braids or laminates thermally linking joints to a refrigerator, or by attaching superconducting joints to a cooled component using an electrically isolating adhesive layer. This latter approach is described, for example, in US 2009/0101325A1.
A difficulty with this latter option arises in achieving sufficient electrical isolation while maintaining adequate thermal conduction for effective cooling of superconducting joints. This generally leads to multiple interfaces between cooled component and superconducting joint, as may be seen in some of the examples described in GB 2453734.
Another approach, in which a superconductor joint is formed in thermal contact with a cooled component, but separated therefrom by an electrically isolating layer, is described in GB 2481833.
That document proposes improved superconducting joints and improved methods for forming superconducting joints in which only a single electrically isolating coating is positioned between the superconducting joint and the cooled component. The electrically isolating coating may be thinner, and is more thermally conductive, than the electrically isolating layers previously employed